Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

3.3. Density-Wave Galaxies

Radio polarization observations show that the regular magnetic fields follow approximately the optical spiral structure in M81 (Krause et al 1989b), M83 (Neininger et al 1991, Ehle 1995), NGC 1566 (Ehle et al 1996), and M51 (Neininger 1992;, Neininger & Horellou 1996; EM Berkhuijsen et al, in preparation), but that the streamlines of the rotation models of the gas do not follow the optical spiral structure. The streamlines have strongly varying pitch angles (e.g. Otmianowska-Mazur & Chiba 1995) and are almost closed in the disk. However, some regions of M51 may be exceptional: Neininger (1992) has claimed that some field lines are carried along with streaming motions. The field lines in the central region of M83 are aligned with the bar.

Strong shocks should compress the magnetic field and lead to high degrees of polarization of 40-70% in the radio continuum (Beck 1982) at the inner edges of the spiral arms (see Section 8.4). Only in M51 are the strongest aligned fields indeed found at the positions of the prominent dust lanes on the inner edges of the optical spiral arms (Figure 1). This is best visible along the eastern arm where the aligned field even follows the dust lane crossing the optical arm. However, some regular fields extend far into the interarm regions. Furthermore, the 10-30% polarization at 6 cm is in contrast to the higher polarizations expected from shock alignment. Hence the radio data only tell us that the regular fields in M51 are somehow coupled to the cool gas as traced by dust lanes.

Figure 1. Polarized synchrotron intensity (contours) and magnetic field orientation of M51 (obtained by rotating the E-vectors by 90°), observed at 6.2 cm with the VLA (12 arcsec synthesized beam). (From Neininger & Horellou 1996.)

The aligned fields in M81 and NGC 1566 are strongest in interarm regions (Krause et al 1989b, Ehle et al 1996), whereas the total synchrotron intensity (tracing the total field) is highest in the optical spiral arms. Strongly aligned interarm fields have also been detected in the outer parts of M83, where the star formation rate is low (Allen & Sukumar 1990). High-resolution observations of M81 (Schoofs 1992;) see Figure 2) confirmed that the regular fields extend across almost the entire interarm region, but are somewhat stronger near the inner edge of the prominent western spiral arm, where some dust clouds are visible. We stress that the distribution of magnetic pitch angles exhibits a weaker arm-interarm variation than that of the regular magnetic field strengths. Soida et al (1996) showed that strength and pitch angle of the regular fields in NGC 4254 reveal much less arm-interarm variations than expected from density-wave compression in its two major arms. They also showed that regular fields even exist in regions of chaotic optical pattern.

Figure 2. Polarized synchrotron intensity (contours) and magnetic field orientation in the south-western part of M81 (obtained by rotating the E-vectors by 90°), observed at 6.2 cm with the VLA (25 arcsec synthesized beam). The circle indicates the half-power diameter of the primary beam. (From Schoofs 1992.)